Camshaft phasers for varying the valve timing of internal combustion engines are well known. A phaser typically comprises a rotor element attached to the end of a camshaft and variably displaceable rotationally within a stator element driven by the engine crankshaft. Phasers typically are actuated by a pressure duty cycle of oil derived from the engine's main oil supply and selectively directed to chambers within the phaser to alter the phase relationship between the rotor and the stator, and hence between the camshaft and the crankshaft.
Modern engines commonly are provided with a phaser on the intake valve camshaft; however, it is known that still greater engine control and efficiency can be obtained by providing an additional independent phaser on the exhaust valve camshaft in an engine having separate camshafts for separately actuating the intake valves and the exhaust valves. V-style engines may have two intake and two exhaust camshafts, for a total of four phasers.
A problem in operating an engine equipped with dual independent camshaft phasers (DICP) is how to coordinate their operation. In the prior art, control methods cannot deal with both phasers simultaneously, but rather with first one phaser and then the other, in rapid alternating succession or with both moving simultaneously, resulting in limited coordination of the two phasers.
Further, large demands are placed on the oiling system of the engine during transient conditions wherein the two (or more) phasers must be actuated during the same time period. Providing oiling system capacity to insure acceptable simultaneous actuation performance at full phasing rate adds cost to the engine, reduces fuel economy, and may result in aeration of the oil by drawing air from the engine crankcase. Additionally, fast step changes in phase position can result in combustion and air/fuel control issues due to inconsistent movement of the cam phasers with respect to time and to each other. Prior art hydraulically actuated cam phaser systems are susceptible to part-to-part, engine-to-engine, and even event-to-event variation that can be detrimental to combustion, air/fuel ratio control, driveability, emissions, and vehicle performance.
What is needed in the art is a system for controlling dual independent camshaft phasers (intake and exhaust) to provide simultaneous ratiometric scheduling of the phasers' movements relative to each other.
It is a principal object of the invention to provide controlled, consistent, synchronous movement of dual independent camshaft phasers, each operating at a rate within the constraints of the engine oiling system, to maintain combustion stability.